feat: add pkg (FreeBSD) and ports (OpenBSD) package manager implementations

pkg implements Manager, VersionQuerier, Cleaner, and FileOwner interfaces
wrapping FreeBSD's pkg(8) CLI. ports implements Manager wrapping OpenBSD's
pkg_add/pkg_delete/pkg_info tools.

Both use build tags (freebsd/openbsd) for real implementations with stub
files for other platforms. Includes parser tests for all output formats.

ports/ports_other.go

@@ -0,0 +1,51 @@
+//go:build !openbsd
+
+package ports
+
+import (
+	"context"
+
+	"github.com/gogrlx/snack"
+)
+
+func available() bool { return false }
+
+func install(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func remove(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func purge(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func upgrade(_ context.Context, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func update(_ context.Context) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func list(_ context.Context) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func search(_ context.Context, _ string) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func info(_ context.Context, _ string) (*snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func isInstalled(_ context.Context, _ string) (bool, error) {
+	return false, snack.ErrUnsupportedPlatform
+}
+
+func version(_ context.Context, _ string) (string, error) {
+	return "", snack.ErrUnsupportedPlatform
+}